Large quantities of methane, the main component of natural gas, are available in many areas of the world, and natural gas is predicted to outlast oil reserves by a significant margin. However, most natural gas is situated in areas that are geographically remote from population and industrial centers. The costs of compression, transportation, and storage make its use economically unattractive. To improve the economics of natural gas use, much research has focused on the use of methane as a starting material for the production of higher hydrocarbons and hydrocarbon liquids, which are more easily transported and thus more economical. The conversion of methane to hydrocarbons is typically carried out in two steps. In the first step, methane is converted into a mixture of carbon monoxide and hydrogen (i.e., synthesis gas or syngas). In a second step, the syngas is converted into hydrocarbons.
This second step, the preparation of hydrocarbons from synthesis gas, is well known in the art and is usually referred to as Fischer-Tropsch synthesis, the Fischer-Tropsch process, or Fischer-Tropsch reaction(s). Fischer-Tropsch synthesis generally entails contacting a stream of synthesis gas with a catalyst under temperature and pressure conditions that allow the synthesis gas to react and form hydrocarbons.
More specifically, the Fischer-Tropsch reaction entails the catalytic hydrogenation of carbon monoxide to produce any of a variety of products ranging from methane to higher alkanes and aliphatic alcohols. The reaction is carried out by contacting the hydrogen and carbon monoxide with a catalyst. The reaction gives off a large amount of heat. When the Fischer-Tropsch reaction is carried out in fixed-bed reactors, this high heat of reaction results in an increase in the temperature of the catalyst bed above that of the surrounding environment. Excessive temperature rises can lead to inferior product distribution, and can damage the catalyst if not controlled.
When the Fischer-Tropsch process is carried out in a fixed bed reactor, synthesis gas is fed via an inlet into direct contact with a catalyst located inside catalyst tubes, while heat is removed from the catalyst bed through catalyst tube walls to a heat exchange medium outside the catalyst tubes. The heat exchange medium may be water. As previously described, the catalyst is typically contained in one or more tubular conduits and the heat exchange medium is located in the spaces between the catalyst tubes. The optimum temperature gradient between the catalyst and the heat exchange medium must be such that the catalyst produces a product having the desired spectrum of hydrocarbons while the catalyst bed remains thermally stable.
Slurry bed reactors allow operators to maintain a more uniform temperature profile along both axial and radial directions of the reactors than those of fixed bed reactors. Also, in slurry bed reactors the heat transfer properties are better than in fixed bed reactors, which leads to better temperature control, an important parameter for exothermic reactions (i.e. Fischer-Tropsch reactions).
The hydrocarbons produced in the Fischer-Tropsch process range from single-carbon methane gas, up to C50 and higher. Because some of the produced hydrocarbons are liquids at the Fischer-Tropsch reactor conditions, there is a need to continuously remove the product from the reactor by separating the liquid from the solid catalyst particles in the slurry. This operation is difficult and expensive to implement. Furthermore, during the separation process, there is a high probability of catalyst attrition, which, in turn, is detrimental to the separation process and may cause loss of catalyst from the reactor and contamination of the products, with negative effects for the processes downstream from the Fischer-Tropsch reactor.
Hence, there remains a need for a catalyst system that provides good heat transfer and thermal control capabilities while minimizing catalyst attrition and avoiding the need for liquid/solid separation equipment.